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An assessment of the influence of the cooling rate in the Al-Si order of K-feldspar of the Hercynian Tábua porphyritic biotite granite, in central Portugal, is presented, taking into account the results of a numerical cooling model. As estimated from cel l- parameter data, ca. 92% Al is located in T1 sites of the K-feldspar throughout the whole pluton, except in a 1.5-km-wide margin, where this occupancy rapidly decreases to ca. 79% close to the contact. Although below ca. 500°C, cooling rate was fairly uniform over the entire pluton, in the range 650-500°C it was, on average, 2°C/ka in the margin and 0.6°C/ka in the interior. Cooling rate of the pluton and degree of order of K-feldspar are highly correlated. In our opinion, cooling rate was the determ in- ing factor in establishing the Al-Si order of K-feldspar over most of the pluton. However, some observations point to reversal of the effect of cooling rate near the contact, in areas of pegmatite-bearing granite, where a fluid phase presumably had a major role.

THE ROLE OF COOLING RATE ON THE Al Si ORDER OF K-FELDSPAR IN THE HERCYNIAN TABUA GRANITE, CENTRAL PORTUGAL